The present invention relates to apparatus for demodulating information from a radio communications carrier signal and, more particularly, to novel non-coherent demodulators providing improved data detection performance for received signals having information modulated thereon by angle modulation, such as narrow band frequency modulation, techniques.
It is well known that transceivers for the radio communication of information by analog voice techniques, such as is used in the conventional Land Mobile Radio services, advantageously utilize narrow band frequency modulation (FM) techniques. In such FM transceivers, the received signal is often converted to an intermediate frequency (IF) and the information thereon is demodulated using a quadrature detector. Further, data communication links which may have transmission of a "burst", or short durational, nature require that the receiver section, of any transceiver utlized in the system, by able to rapidly acquire the transmitted signal. The rapid acquisition requirement is typically satisfied by utilizing non-coherent demodulator techniques. In conventional Land Mobile Radio systems utilizing narrow band FM, the non-coherent demodulator is usually implemented as a quadrature detector, which performs as a time-differentiator by producing a voltage which is proportional to the received signal instantaneous frequency. Thus, the output of the quadrature detector is recovered data which has been frequency modulated onto the transmitted carrier. However, since the quadrature detector is deliberately made sensitive to the received signal frequency, any frequency offset from the nominal carrier frequency will distort the recovered data. While it is possible to reduce the effect of frequency offset for the quadrature detector, this imposes an unreasonable frequency stability requirement on the overall communications link. Consequently, the quadrature detector is configured to be a compromise between data detection peformance and received signal acquisition range.
These same transceivers are often utilized with other, more modern techniques for digital data and digital secure voice transmissions, wherein the efficacy of the quadrature detector is considerably less than its efficacy for analog voice transmission. In any service where both analog and digital voice and data transmissions may occur, the trade-off of demodulation performance against received signal frequency acquisition is a major disadvantage of a quadrature detector and may result in system performance which can be significantly less than desired. It is highly desirable that the demodulator operate upon many forms of carrier signals having what we term "angle", or non-amplitude, modulation; this includes frequency modulations such as narrowband frequency modulation (FM) and continuous-phase frequency shift keying (CPFSK) and phase modulations such as phase shift keying (PSK), either as of binary BPSK or of M-ary MPSK nature. Accordingly, we desire to provide a non-coherent demodulator which receives angle-modulation signals, possibly having a substantial frequency offset, and detects digital data/voice information therefrom.